Firings At Out of Eden, Wales : August 2019
This page is an archive of firings carried out by Bill Crumbleholme with the assistance of Mark Vyvyan-Penny at the Neolithic settlement Out of Eden in Wales.
The wonderful team of folk at the settlement had invited Bill and Mark along to demonstrate their techniques and manage workshops.
More can be found out at the website www.thewildernesstrust.org/out-of-eden/
The first firing was done using an open bonfire, the second was in a kiln structure built by Mark and the team. The images and text below are an archive of the firings, with a section between about the preparation of clay and making of pottery.
Bonfire Firing
The pots were made by Bill several weeks before, now nice and dry. They were a batch of bowls made in the style of Neolithic wares. Most are round bottomed.
The bowls were laid out around a small fire, with another ring of fire around them. Without the outer ring of fire the pots would suffer from a draught of cold air being drawn over them as the hot air rises from the central fire. The outer fire also warms the pots.
The fires are kept small, gradually being spread around the ring. About six inches away from the pots, to preheat them with radiant and convected heat, for several hours. If a flame licks up the side of a pot it can spall, where the water just under the surface is superheated and turns to steam with a rapid expansion that blows a section of the face of the pot away.
The slow heating dries out the pots and they change colour and texture, becoming more porous as the dampness is driven out. Those open pores allow water to escape as it turns to steam. When tapped with a stick the sound is tinnier.
After 4 hiours of preheating, when considered dry enough, the fire had been allowed to die back, the pots were rearranged into a pair of rows. During that procedure the largest bowl suffered a spalling explosive break, probably because part of it had not been sufficently pre-heated.
Two large branches were placed alongside the pots and then small branches were placed across over the pots. The flames then started to revive.
The flames slowly built up, into a good blaze.
After a hour or so the stoking was over and the fire allowed to die away, the pots start to appear out of the embers , glowing red - which means they were hot enough to turn ceramic and would not turn back to soft clay when wetted. About 750C is a good temperature to reach.
Much of the neolithic pottery analysed contains sea-shell temper - crushed shells were added to the clay to enhance its properties. Calcium carbonate (which is what sea shell consists of mainly) undergoes a chemical breakdown and is turned to quick lime at just over 800C, that is catestrophic for the pot as it rehydrates after the firing and expands and turns slightly liquid, which ruins the small part of the pot affected.
The pots were left overnight to cool down.
Next day the pots were revealed. Some had not cracked!
But some had cracked.
Bill lifting out the pots.
The pots were slightly smudged with soot, about half had small cracks in the rims, a couple had spalling breaks and one broke.
Clay preparation and Pot making
During the other activities, batches of clay and inclusions were prepared and pots were made by the volunteers and visitors.
Bill had brought some clay from Dorset, which had been prepared in various ways and stages - eat your heart out Blue Peter "Here's some I prepared earlier!"
Batches of clay dug locally were also processed.
Most contemporary commercially prepared clays are a blend of several clays, which each provide certain properties to make a better clay than any of them alone. Ancient potters would have understood this and probably used blends, unless they were fortuneate enough to locate a seam of clay that worked well by itself. They might have brought clays by river when attending gatherings where feasting needed large pots for cooking, rather than trying to transport fragile vessels.
None of the clays available provided a raw material that was much good for making pottery! But it was found that a blend of local clay and Dorset clay worked better than either alone.
When making pots it is easier if the clays are elastic and plastic by nature, so that they can be moulded into shape by pinching etc without too many cracks appearing, especially around the rims. Most clays can cope with compression - being squeezed into shape, but most are unable to withstand tension - being stretched without tearing or bursting. This is helped by having some much finer clay partitles from the geological erosion activities undergone. It is hindered by the larger pieces of temper which form focus points for the making stresses to produce cracks around them.
The Dorset clay was from seams of Oxford Clay close to the sites of old brickworks, it fired satisfactorily and did not have any impurities that caused too much grief - apart from some small garden pea sized nodules.
Those nodules of gravel or whatever were removed in two ways :-
By sorting carefully by hand, smearing the clay thin enough for them to be found, that also helped to break down the lumps of harder clay and could be part of the method of then introducing the tempers.
Bill had previously done the second approach, which was to wet down the clay into a slurry and pass it through a garden sieve to catch impurities. Some dry tempers were added to the wet clay which helped stiffen them up, they were then dried on plaster and bagged up.
An assortment of tempers were prepared on site. These were all materials found in neolithic pottery. They are used to enhance the properties of the clay, at all stages - during shaping, firing and use - by making the clay body more resiliant to thermal shock, more porous to enable steam to escape when being fired and to assist with a flameproof nature in use.
Ancient tempers may have had an added property of being derived from culturally important sources or artefacts. Thus sea shell may have been associated with a feasting event. Flint may have been a byproduct of tool production, maybe from a culturally special geological source - or even an old tool crushed and repurposed. Grog, which is crushed and ground up fired pottery, may have been from mis-fired pottery, but was also likely to be from used pots which were then given "rebirth" when recycled - perhaps when the owner had died.
Some tempers had been pre-heated to make them easier to grind, seashells became easier to crush and grind into smaller pieces, flint also lost some sharpness. The prepheating was done in a metal biscuit tin over a gas kiln, but this could have been done on a fire within a more authententic pot.
The crushing and grinding was undertaken by volunteers, using a large flat stone with a large beach pebble as the tool. Various methods were tested, hammering to crush followed by a rolling motion to grind the materials seemed to work well. A collender was used to sort the materials - with larger pieces reground. Ancient pots have been found with holes drilled in the bases, which might have been used for this process (as well as the usual understanding of cheese production etc).
The clay and tempers were mixed using one of Bill's experimental methods, the clay was formed into blocks which were then sliced with a wire (fresh out of animal sinue!) into slabs about 5mm thick. Those slabs were then built up in new layers like a lasange, with the tempers placed between the slabs. Different stiffnesses of clay and dry (or wet) tempers can be blended together to achieve the required final stiffness for pot making.
Downward pressure then flattened out the new block, which was then cut into 4 quarters that were piled on top of each other and flattened again, each flattening thinned out the slabs of clays and tempers. The blocks were then wedged up, by kneading the clay to spirally mix it together.
Volunteers attempted to make pots using the prepared clays, getting an idea of how the various blends and stiffnesses affected the ease of building the vessel.
Bill is not a fan of coil building and teaches making "thumb pots", where a lump of clay is hollowed out using the thumbs (of which he has a very large pair!) and fingers to pinch out the shapes. Occassional "strangling" of the shape keeps it smaller diameter and thus taller and thinner, it also recompresses the clay and helps close up any cracks that might have started appearing.
Depending on the nature of the clay and the dryness of the weather and the amount of handling, over time the clay dries out and becomes more rigid, so it can then be made thinner without collapsing under its own weight - literally going pear shaped!
Cracks usually appear as the clay is pinched into shape, as it is very difficult to stop it from being stretched even slightly as the pinching results in stretching in the adjacent area, especially atroiund the rim of the pot. These cracks can be minimised by brushing on a little water or better still slip (a very wet clay slurry), that soaks into the clay and when compressed together the cracks join back together with the water acting as a glue.
Surface cracks and bumps can be smoothed out by applying a little water or slip and when soaked in a piece of wood or bone with an edge can be used to scrape the surface, removing bumps and forcing wet clay into the cracks. That may need to be repeated several times, with a rest between to allow the pot to dry out a little. Too much damp will make the pot collapse as it gets soggy!
It is important to make the walls and base of the pot a similar thickness throughout, to minimise the stress set up by shinking as it dries - thick sections taking longer to dry and shrink. The join between the wall and base is a particularly tricky part of the pot to get correct thickness, which is one reason why rounded bottom bowls are stronger, as it is easier to feel the evenness of the pot.
Constructed Kiln Firing
The base was excavated through the turf about a foot down into the clay-rich soil. The main chamber was circular with a flat base. Two opposing fire boxes were dug to enable the preheating to be acheived by having fires in them, from which heated air entered the chamber and warmed the pots slowly.
Mark and various volunteers constructed the kiln walls using a mixture of clay, soil, hay and water, rolled into cylinders which were placed in position and joined to the one below by smearing the clay downwards or upwards across the join. A gentle narrowing of the diameter of the chamber roof formed a sort of chimney.
A small fire was burnt in the chamber to slightly dry out the base and walls.
Mark checked the firebox construction, a series of wicker fronds bent over them were covered with the clay mixture on the top and underneath - which slowed down the burning of them so the wall could harden before they stopped being a useful support.
Afteer the initial fire had been scooped into the fireboxes, the pots were placed into the chamber, on a bed of broken pottery - which kept them off the base to avoid cool spots during the later stages of firing
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The fires were gently built up in both fireboxes, preheating the pots and drying them and the kiln structure out over a period of several hours.
One of the grooved ware replicas brought along by Bill can be seen in the chamber viewed through the firebox.
The fires were gradually built up to raise the temperature in the chamber to about 120C.
The "Fire Maidens" inspect the pots through the chimney. A log had been placed at the entrance of the firebox to provide more support for the roof.
The pots became blackened by soot and drier - almost ready for the big burn.
The fires were built up and flames were allowed to come into the main chamber. No pots appeared to spall at this critical point of the process.
After supper the fires were stoked again, this time by lowering branches into the kiln chamber from above. The fireboxes were not stoked, but became the air inlets, to enable the fuel in the chamber to burn efficiently - if the fire boxes were stoked there would not be enought oxygen avaiable in the kiln.
Kevin, although appearing to be carrying out some mystic rite, is gently encouraging the embers to fall down amongst the pots, before the next stoking. Those embers heat up the bases of the pots and minimise the problem of them not getting hot enough to turn ceramic.
The top view into the chamber shows pots filled with glowing embers.
Bill and Mark had to leave after the firing, so Bill used long tongs to lift out one of his grooved ware urns, while still very warm. The rest of the pots were left to cool overnight (images to follow!)
This is the urn Bill removed and took home. It appears to be well fired and reasonably good quality. Unfortunatly the top rim was damaged by the tongs lifting it and small piece broke off.